I set up a site with the ArrayShift modules. I had to go through at least two or three cycles of updating a few of them, and although it created views to display most recent/unanswered/hot/etc questions, it didn't automatically install a new node type for them. It was frustrating trying to figure that out. (The correct type/machine-readable name for the content type is question.)
There's no link in the banner menu to allow you to ask a question like the SE sites have (set in the primary links), but that's aping the SE look and feel too much.
It switches the description for titles on forms for creating content to What's your programming question? Be descriptive. regardless if it's actually a question or not.
A book page. What's your programming question? Be descriptive.
A blog entry. What's your programming question? Be descriptive.
A forum topic. What's your programming question? Be descriptive.
It's a tad annoying.
But anyway I've just started playing around with it and the basic functionality is there. There's some things I'll change to suit my preferences for a Q&A site, but I'll say ArrayShift looks pretty useful (once modules are updated) so far.
EDIT: Putting in a few questions & trying to answer them, it seems that answers are given using Drupal's comment system, rather than a separate answer node. While you have comments for answers in that the comment system permits threading and what-not, it doesn't keep comments for the ? (which are often necessary as you can see in the typical SE ?) together with them, visually expressing their relationship.
Also, I haven't been able to figure out how to activate the markdown editor.